The safety valve is one of the three indispensable safety accessories on the boiler. It plays an important role in the control of the limit value of the internal pressure of the boiler and the safety protection of the boiler. When the pressure in the boiler rises above the specified value, the safety valve automatically opens to relieve pressure and discharge steam; when the pressure in the boiler drops to the specified value, the safety valve automatically closes. If the safety valve is improperly selected or the adjustment and inspection does not meet the requirements of use, it is easy to cause the overpressure of the boiler and even the explosion accident. Therefore, the correct understanding and use of the safety valve is an important part of the boiler safety management.
1. Structure management
(1) Spring-type safety valve: The spring-type safety valve is mainly composed of valve seat, valve core, valve stem, spring, guide sleeve, adjusting screw, lifting handle, valve body and other components. It uses the pressure of the spring acting on the valve core to balance the supporting force of the steam pressure acting on the valve core. When the supporting force of the steam pressure acting on the valve core is greater than that of the spring acting on the valve core, the spring is compressed, the valve core is lifted off the valve seat, and the steam is discharged outward; when the supporting force is smaller than the elastic force, the spring is extended, Press the valve core down until it is tightly combined with the valve seat, and the steam stops discharging outward.
The spring-type safety valve controls the exhaust pressure by adjusting the tightness of the spring. This safety valve has a compact and lightweight structure, and can be vibrated without leaking. It is a commonly used safety valve in boilers at present. However, since the elasticity of the spring is affected by factors such as temperature and time, the reliability is poor, and the safety valve must be inspected regularly during use.
(2) Lever type safety valve: The lever type safety valve is composed of valve body, valve disc, valve seat, valve stem and heavy hammer. It uses the action of a lever and a heavy hammer to compress the valve disc on the valve seat. When the pressure of the steam acting on the valve disc exceeds the pressure acting on the valve stem by the heavy hammer, the valve disc is lifted, and the steam passes through the safety valve. When the steam is discharged, the steam pressure decreases; when the steam pressure acting on the valve disc is less than the weight pressure acting on the valve stem, the valve disc is re-compressed on the valve seat, so that the boiler can continue to operate. The opening pressure of the lever type safety valve is controlled by adjusting the distance between the weight and the fulcrum. After the opening pressure of the safety valve is determined, the distance of the fulcrum of the heavy hammer can be calculated by using the lever principle, and the position of the heavy hammer can be determined. The lever-type safety valve has a simple structure, is easy to adjust, and has a live action recording, which is relatively accurate and reliable. However, the heavy hammer is relatively heavy, the exhaust capacity of a single safety valve is limited, and the installation requirements are also relatively strict.
2. Safety valve technical requirements
The selection of safety valve should meet the requirements of use. First of all, it is necessary to understand the specifications and models of the safety valve. When using and identifying, pay special attention to whether the material of the sealing surface, the material of the valve body and the nominal pressure meet the requirements, as well as problems such as sealing and adhesion. Generally used for safety valve of steam. The valve body material is carbon steel, and the valve seat material is stainless steel.
The nominal pressure of the safety valve requires a reference temperature of 120°C and a steel valve body of 200°C. When the boiler safety valve medium exceeds 200℃, the maximum allowable working pressure is lower than the nominal pressure. Therefore, when the saturated steam pressure exceeds 1.47MPa and the boiler with superheater, the safety valve should be selected according to the working temperature.
Number of safety valves. For boilers with rated evaporation >0.5t/h, at least two safety valves (excluding economizer safety valves) shall be installed. For boilers with evaporation capacity ≤0.5t/h, at least one safety valve shall be installed. Safety valves must be installed at the outlet (or inlet) of the separable economizer and at the outlet of the steam superheater.
The safety valve should be installed perpendicular to the ground, and should be installed at the higher position of the drum and header as much as possible. Between the safety valve and the drum or between the safety valve and the header, no outlet pipes and valves for taking steam shall be installed.
Verification of the opening pressure of the safety valve. The opening pressure of the safety valve should be adjusted and calibrated according to the value specified in the table. For boilers using only one valve, the opening pressure of the safety valve shall be adjusted according to the lower value in Table 1.2-2; for boilers with superheater, the safety valve on the superheater shall be adjusted according to the lower pressure; for boilers with separable economizers , the opening pressure of the safety valve on the economizer is 1.1 times the working pressure of the installation site. For boilers with economizers and superheaters, the opening sequence of their safety valves is: first, the superheater safety valve, then on the drum, and finally on the economizer, in order to comply with the safety performance of components that ensure high working temperature. good principle.
The lever type safety valve must have a device to prevent the weight from moving by itself and a guide frame to limit the lever from going out. The spring-loaded safety valve shall have a lifting handle and a device to prevent the adjustment screw from being twisted indiscriminately.
Connection of safety valve. If several safety valves are jointly installed on a short pipe directly connected to the drum, the passage cross-sectional area of the short pipe should not be less than 1.25 times the cross-sectional area of all safety valves. For boilers with working pressure ≤3.82Mpa, the inner diameter of the safety valve seat should not be less than 25mm.
The safety valve should generally be equipped with an exhaust pipe, and the exhaust pipe should go straight to the outside as much as possible, and have enough cross-sectional area to ensure smooth exhaust. The bottom of the exhaust pipe of the safety valve should be equipped with a drain pipe connected to a safe place. Valves are not allowed to be installed on the exhaust pipe and the drain pipe. The safety valve of the economizer should be equipped with a drain pipe and lead to a safe place. No valve shall be installed on the drain pipe.
Periodically do a gas or water test. In order to prevent the valve disc and valve seat of the safety valve from sticking, the safety valve should be periodically tested for manual or automatic deflation or water release.
Administration and usage precautions. After the safety valve has been calibrated, it should be locked or sealed. It is strictly forbidden to add heavy objects, move the heavy hammer, or jam the valve disc, so as to prevent the opening pressure of the safety valve from being arbitrarily increased or the safety valve becoming invalid.